In active matrix substrates, a thin film transistor (hereinafter also referred to as the “TFT”), for example, is provided as a switching element in each pixel as a minimum unit of an image.
Typical bottom gate TFTs include, e.g., a gate electrode provided on an insulating substrate, a gate insulating film provided so as to cover the gate electrode, a semiconductor layer formed in an island shape on the gate insulating film so as to overlap the gate electrode, and source and drain electrodes provided on the semiconductor layer so as to face each other.
In recent years, in active matrix substrates, TFTs using a semiconductor layer of an oxide semiconductor (hereinafter also referred to as the “oxide semiconductor layer”) comprised of an In—Ga—Zn—O (IGZO) oxide semiconductor film with high mobility have been proposed as switching elements of pixels as a minimum unit of an image, instead of conventional thin film transistors using a semiconductor layer of amorphous silicon.
More specifically, for example, a thin film transistor substrate has been disclosed which includes an insulating layer, a gate electrode provided on the insulating layer, an oxide semiconductor layer provided on the gate electrode with a gate insulating film interposed therebetween, source and drain electrodes provided on the oxide semiconductor layer so as to be separated from each other with the gate electrode interposed therebetween, and a channel protective film provided between the source and drain electrodes and the oxide semiconductor layer so as to cover at least a part of a side surface of the oxide semiconductor layer which is exposed from the source and drain electrodes over the gate electrode.
In order to manufacture this thin film transistor substrate, the steps of forming a gate electrode on a substrate, forming a gate insulating film on the gate electrode, forming an oxide semiconductor layer on the gate insulating film, and forming a channel protective film so as to cover at least a part of a side surface of the oxide semiconductor layer over the gate electrode are first performed. Next, the step of heating the oxide semiconductor layer and the channel protective film at a temperature of 160° C. or higher, and then forming source and drain electrodes on the oxide semiconductor layer and the channel protective film is performed. The thin film transistor substrate is manufactured by using five photomasks (see, e.g., Patent Document 1).